↓ Period 4 5 6 7 Legend primordial element element by radioactive decay synthetic element
Group 7, numbered by IUPAC nomenclature, is a group of elements in the periodic table. It contains manganese (Mn), technetium (Tc), rhenium (Re) and bohrium (Bh). This group lies in the d-block of the periodic table, and are hence transition metals. This group is sometimes called the manganese group or manganese family after its lightest member; however, the group itself has not acquired a trivial name because it belongs to the broader grouping of the transition metals.
The group 7 elements tend to have a major group oxidation state (+7), although this trend is markedly less coherent than the previous groups. Like other groups, the members of this family show patterns in their electron configurations, especially the outermost shells resulting in trends in chemical behavior. In nature, manganese is a fairly common element, whereas rhenium is rare, technetium only occurs in trace quantities, and bohrium is entirely synthetic.
Physical properties
The trends in group 7 follow, although less noticeably, those of the other early d-block groups and reflect the addition of a filled f-shell into the core in passing from the fifth to the sixth period. All group 7 elements crystallize in the hexagonal close packed (hcp) structure except manganese, which crystallizes in the body centered cubic (bcc) structure. Bohrium is also expected to crystallize in the hcp structure.
The table below is a summary of the key physical properties of the group 7 elements. The properties of bohrium are either unknown or predicted, as they have not been measured.
Properties of the group 7 elements Name Mn, manganese Tc, technetium Re, rhenium Bh, bohriumMelting point 1519 K (1246 °C) 2430 K (2157 °C) 3459 K (3186 °C) Boiling point 2334 K (2061 °C) 4538 K (4265 °C) 5903 K (5630 °C) Density 7.21 g·cm−3 11 g·cm−3 21.02 g·cm−3 26-27 g·cm−3?Appearance silvery metallic silvery-gray silvery-gray Atomic radius 127 pm 136 pm 137 pm 128 pm?
Chemical properties
Like other groups, the members of this family show patterns in its electron configuration, especially the outermost shells:
Z Element Electrons per shell 25 manganese 2, 8, 13, 2 43 technetium 2, 8, 18, 13, 2 75 rhenium 2, 8, 18, 32, 13, 2 107 bohrium 2, 8, 18, 32, 32, 13, 2
All the members of the group readily portray their group oxidation state of +7 and the state becomes more stable as the group is descended. Technetium also shows a stable +4 state whilst rhenium exhibits stable +4 and +3 states.
Bohrium may therefore also show these lower states as well. The higher +7 oxidation state is more likely to exist in oxyanions, such as perbohrate, BhO4−, analogous to the lighter permanganate, pertechnetate, and perrhenate. Nevertheless, bohrium(VII) is likely to be unstable in aqueous solution, and would probably be easily reduced to the more stable bohrium(IV).
Compounds
Oxides
Manganese
Manganese forms a variety of oxides: MnO, Mn3O4, Mn2O3, MnO2, MnO3 and Mn2O7. Manganese(II) oxide is an inorganic compound that forms green crystals. Like many monoxides, MnO adopts the rock salt structure, where cations and anions are both octahedrally coordinated. Also like many oxides, manganese(II) oxide is often nonstoichiometric: its composition can vary from MnO to MnO1.045. Manganese(II,III) oxide is formed when any manganese oxide is heated in air above 1000 °C. Considerable research has centred on producing nanocrystalline Mn3O4 and various syntheses that involve oxidation of MnII or reduction of MnVI.Hausmannite Mn3O4 nanorods: synthesis, characterization and magnetic properties Jin Du et al. Nanotechnology, (2006),17 4923-4928, One-step synthesis of Mn3O4 nanoparticles: Structural and magnetic study Vázquez-Olmos A., Redón R, Rodríguez-Gattorno G., Mata-Zamora M.E., Morales-Leal F, Fernández-Osorio A.L, Saniger J.M. Journal of Colloid and Interface Science, 291, 1, (2005), 175-180 Manganese(III) oxide is unlike many other transition metal oxides in that it does not adopt the corundum (Al2O3) structure. Two forms are generally recognized, α-Mn2O3 and γ-Mn2O3,Wells A.F. (1984) Structural Inorganic Chemistry 5th edition Oxford Science Publications although a high pressure form with the CaIrO3 structure has been reported too.High Pressure Phase transition in Mn2O3 to the CaIrO3-type Phase Santillan, J.; Shim, S. American Geophysical Union, Fall Meeting 2005, abstract #MR23B-0050 Manganese(IV) oxide is a blackish or brown solid occurs naturally as the mineral pyrolusite, which is the main ore of manganese and a component of manganese nodules. The principal use for MnO2 is for dry-cell batteries, such as the alkaline battery and the zinc–carbon battery. Manganese(VII) oxide is dark green in its crystalline form. The liquid is green by reflected light and red by transmitted light. It is soluble in carbon tetrachloride, and decomposes when in contact with water.
Technetium
Technetium's main oxides are technetium(IV) oxide and technetium(VII) oxide. Technetium(IV) oxide was first produced in 1949 by electrolyzing a solution of ammonium pertechnetate under ammonium hydroxide. It has often been used to separate technetium from molybdenum and rhenium. More efficient ways are the reduction of ammonium pertechnetate by zinc metal and hydrochloric acid, stannous chloride, hydrazine, hydroxylamine, ascorbic acid, by the hydrolysis of potassium hexachlorotechnetate or by the decomposition of ammonium pertechnetate at 700 °C under an inert atmosphere. It reacts with oxygen to produce technetium(VII) oxide at 450 °C.
Technetium(VII) oxide can be prepared directly by the oxidation of technetium at 450-500 °C. It is a rare example of a molecular binary metal oxide. Other examples are ruthenium(VIII) oxide and osmium(VIII) oxide. It adopts a centrosymmetric corner-shared bi-tetrahedral structure in which the terminal and bridging Tc−O bonds are 167pm and 184 pm respectively and the Tc−O−Tc angle is 180°.
Rhenium
Rhenium's main oxides are rhenium(IV) oxide and rhenium(VII) oxide. Rhenium(IV) oxide is a gray to black crystalline solid that can be formed by comproportionation.G. Glemser "Rhenium (IV) Oxide" Handbook of Preparative Inorganic Chemistry, 2nd Ed. Edited by G. Brauer, Academic Press, 1963, NY. Vol. 1. p. 1480. At high temperatures it undergoes disproportionation. It is a laboratory reagent that can be used as a catalyst. It adopts the rutile structure. It forms perrhenates with alkaline hydrogen peroxide and oxidizing acids. In molten sodium hydroxide it forms sodium rhenate:G. Glemser "Sodium Rhenate (IV)" Handbook of Preparative Inorganic Chemistry, 2nd Ed. Edited by G. Brauer, Academic Press, 1963, NY. Vol. 1. p. 1483.
2NaOH + ReO2 → Na2ReO3 + H2O
Rhenium(VII) oxide can be formed when rhenium or its oxides or sulfides are oxidized a 500-700 °C in air. It dissolves in water to give perrhenic acid. Heating Re2O7 gives rhenium(IV) oxide, signalled by the appearance of the dark blue coloration. In its solid form, Re2O7 consists of alternating octahedral and tetrahedral Re centres. It is the raw material for all rhenium compounds, being the volatile fraction obtained upon roasting the host ore.
Rhenium, in addition to the +4 and +7 oxidation states, also forms a trioxide. It can be formed by reducing rhenium(VII) oxide with carbon monoxide at 200 C or elemental rhenium at 4000 C. It can also be reduced with dioxane. It is a red solid with a metallic lustre that resembles copper in appearance, and is the only stable trioxide of the group 7 elements.
Halides
Manganese
Manganese can form compounds in the +2, +3 and +4 oxidation states. The manganese(II) compounds are often light pink solids. Like some other metal difluorides, MnF2 crystallizes in the rutile structure, which features octahedral Mn centers. and it is used in the manufacture of special kinds of glass and lasers. Scacchite is the natural, anhydrous form of manganese(II) chloride. The only other currently known mineral systematized as manganese chloride is kempite - a representative of the atacamite group, a group of hydroxide-chlorides. It can be used in place of palladium in the Stille reaction, which couples two carbon atoms using an organotin compound. It can be used as a pink pigment or as a source of the manganese ion or iodide ion. It is often used in the lighting industry.
Technetium
The following binary (containing only two elements) technetium halides are known: TcF6, TcF5, TcCl4, TcBr4, TcBr3, α-TcCl3, β-TcCl3, TcI3, α-TcCl2, and β-TcCl2. The oxidation states range from Tc(VI) to Tc(II). Technetium halides exhibit different structure types, such as molecular octahedral complexes, extended chains, layered sheets, and metal clusters arranged in a three-dimensional network. These compounds are produced by combining the metal and halogen or by less direct reactions.
TcCl4 is obtained by chlorination of Tc metal or Tc2O7 Upon heating, TcCl4 gives the corresponding Tc(III) and Tc(II) chlorides. TcCl4 → α-TcCl3 + 1/2 Cl2 TcCl3 → β-TcCl2 + 1/2 Cl2 The structure of TcCl4 is composed of infinite zigzag chains of edge-sharing TcCl6 octahedra. It is isomorphous to transition metal tetrachlorides of zirconium, hafnium, and platinum.
Two polymorphs of technetium trichloride exist, α- and β-TcCl3. The α polymorph is also denoted as Tc3Cl9. It adopts a confacial bioctahedral structure. It is prepared by treating the chloro-acetate Tc2(O2CCH3)4Cl2 with HCl. Like Re3Cl9, the structure of the α-polymorph consists of triangles with short M-M distances. β-TcCl3 features octahedral Tc centers, which are organized in pairs, as seen also for molybdenum trichloride. TcBr3 does not adopt the structure of either trichloride phase. Instead it has the structure of molybdenum tribromide, consisting of chains of confacial octahedra with alternating short and long Tc—Tc contacts. TcI3 has the same structure as the high temperature phase of TiI3, featuring chains of confacial octahedra with equal Tc—Tc contacts.
Several anionic technetium halides are known. The binary tetrahalides can be converted to the hexahalides [TcX6]2− (X = F, Cl, Br, I), which adopt octahedral molecular geometry. More reduced halides form anionic clusters with Tc–Tc bonds. The situation is similar for the related elements of Mo, W, Re. These clusters have the nuclearity Tc4, Tc6, Tc8, and Tc13. The more stable Tc6 and Tc8 clusters have prism shapes where vertical pairs of Tc atoms are connected by triple bonds and the planar atoms by single bonds. Every technetium atom makes six bonds, and the remaining valence electrons can be saturated by one axial and two bridging ligand halogen atoms such as chlorine or bromine.
Rhenium
The most common rhenium chlorides are ReCl6, ReCl5, ReCl4, and ReCl3. The structures of these compounds often feature extensive Re-Re bonding, which is characteristic of this metal in oxidation states lower than VII. Salts of [Re2Cl8]2− feature a quadruple metal-metal bond. Although the highest rhenium chloride features Re(VI), fluorine gives the d0 Re(VII) derivative rhenium heptafluoride. Bromides and iodides of rhenium are also well known.
Like tungsten and molybdenum, with which it shares chemical similarities, rhenium forms a variety of oxyhalides. The oxychlorides are most common, and include ReOCl4, ReOCl3.
Organometallic compounds
Manganese
Organomanganese compounds were first reported in 1937 by Gilman and Bailee who described the reaction of phenyllithium and manganese(II) iodide to form phenylmanganese iodide (PhMnI) and diphenylmanganese (Ph2Mn).